Nitrocarburizing of low-carbon unalloyed steel

Abstract

Low-carbon unalloyed steel has been investigated after nitriding accomplished by either the salt-bath method (Tenifer) or the gas method (Nitroc). The resulting changes in the mechanical properties are reported in this paper, while changes in positron annihilation parameters are reported in Part 2. The mechanical properties show that the increase in both the lower yield stress and the ultimate stress is influenced greatly by the cooling rate after nitriding. A rapid cooling rate resulted in an increase of ∼200 MPa while an increase of only 55 Mpa was observed for a slow cooling rate. These increases proved independent of prior straining in the range 0 to 20%, showing that the nitrogen uptake is not significantly influenced by internal defects. Annealing of rapidly cooled nitrided specimens resulted in a maximum of both the lower yield stress and the ultimate yield stress around 100° C. A transition from brittle to ductile fracture around this temperature was also observed. A large recovery of elongation to fracture was obtained after annealing at 140° C, which was not accompanied by any significant decrease in the lower yield stress. Dilatometer measurements indicated two precipitation stages, one around room temperature and another around 100° C.